Storage allocation strategies

STORAGE ALLOCATION STRATEGIES

Sreel aj

DIFFERENT WAYS TO ALLOCATE OBJECTS 

Static allocation – allocates storage at compile time

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Stack allocation - manages run time storage as stack

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Heap allocation – allocates and de-allocates storage area as needed at runtime.

STATIC ALLOCATION Names are bound to storage locations  This property allows values of local names to be retained across the complete program  At the compile time compiler determines how much storage should be allocated for each object. 

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At the compile time, compiler determines the following

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1 . Where the activation records go, relative to target code 2 . Where the addresses should be filled in the records 3 . The address for the procedure calls

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LIMITATIONS

Size & position of data objects must be known at compile time.  Recursive procedures are restricted.  Data objects cannot be created dynamically. 

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E.g.: Fortran

FORTRAN A Fortran compiler might place the activation record for a procedure together with the code for that procedure.  In some systems it is possible to use link editor to link activation records and executable code. 

STACK ALLOCATION Storage is organized as a stack  Activation records are pushed and popped as activation begin and end  Storage for locals in each call of a procedure is contained in the activation record record for that call  The values of locals are deleted when the activation ends  A register can be used to mark the top of  stack. At run time an activation record can be allocated and de-allocated by incrementing and decrementing register. register. 

CALLING SEQUENCES For implementing procedure calls.  A call sequence allocates an activation a ctivation record and entries information into its fields.  A return sequence restores the state of the machine so the calling procedure can continue execution. 

CALLING SEQUENCES  The code in a calling ca lling sequence is often divided between the calling procedure and the procedure it calls  Usually fixed size fields are placed in the middle  Temporary  Temporary fields are often stored sto red after local loc al data to a procedure so that changes in its size will not affect the offsets of data objects 

DIVISION OF TASKS BETWEEN CALLER

DIVISION OF TASKS BETWEEN CALLER AND CALLEE

PARAMETERS & RETURN VAL Control link

Callers activation record

 Temporaries & local data PARAMETERS & RETURN VAL Control link  Temporaries & local data

Callee’s activation record

 THE CALLING SEQUENCE  The caller evaluates the actuals  The caller stores a return address & old value of stack in its activation & increments pointer pointer to new position  Callee saves register values and other information  Callee initializes its local data & begins execution 

A POSSIBLE RETURN SEQUENCE  The callee places a return value next to activation record of the caller  Using info on activation a ctivation record record , the callee restores stack pointer & other registers & branches to the return address in callers code  The caller can copy the returned value to its own activation record record & use it to evaluate an expression. 

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